The present invention is in the field of upgrading heavy oil feedstocks that include a significant quantity of asphaltenes and fraction boiling above 524° C. (975° F.) into lower boiling, higher quality materials. More particularly, the invention relates to hydroprocessing systems that employ a colloidal or molecular hydroprocessing catalyst that promotes hydrocracking and hydrotreatment of heavy oil feedstocks while also inhibiting fouling of the system by coke precursors and sediment.
2. The Relevant Technology
World demand for refined fossil fuels is ever-increasing and will inevitably outstrip the supply of high quality crude oil, whether as a result of actual shortages or due to the actions of oil cartels. In either case, as the price or shortage of crude oil increases there will be an every-increasing demand to find ways to better exploit lower quality feedstocks and extract fuel values therefrom. As more economical ways to process lower quality feedstocks become available, such feedstocks may possibly catch, or even surpass, higher quality crude oils, in the not-too-distant future, as the primary source of refined fossil fuels used to operate automobiles, trucks, farm equipment, aircraft, and other vehicles that rely on internal combustion.
Lower quality feedstocks are characterized as including relatively high quantities of hydrocarbons that have a boiling point of 524° C. (975° F.) or higher. They also contain relatively high concentrations of sulfur, nitrogen and metals. High boiling fractions typically have a high molecular weight and/or low hydrogen/carbon ratio, an example of which is a class of complex compounds collectively referred to as “asphaltenes”. Asphaltenes are difficult to process and commonly cause fouling of conventional catalysts and hydroprocessing equipment.
Examples of lower quality feedstocks that contain relatively high concentrations of asphaltenes, sulfur, nitrogen and metals include heavy crude and oil sands bitumen, as well as bottom of the barrel and residuum left over from conventional refinery process (collectively “heavy oil”). The terms “bottom of the barrel” and “residuum” (or “resid”) typically refer to atmospheric tower bottoms, which have a boiling point of at least 343° C. (650° F.), or vacuum tower bottoms, which have a boiling point of at least 524° C. (975° F.). The terms “resid pitch” and “vacuum residue” are commonly used to refer to fractions that have a boiling point of 524° C. (975° F.) or greater.
By way of comparison, Alberta light crude contains about 9% by volume vacuum residue, while Lloydminster heavy oil contains about 41% by volume vacuum residue, Cold Lake bitumen contains about 50% by volume vacuum residue, and Athabasca bitumen contains about 51% by volume vacuum residue. Resid contains even higher concentrations of fractions that boil at or above about 343° C. (650° F.), with vacuum tower bottoms almost exclusively comprising fractions that boil at or above about 524° C. (975° F.).
Converting heavy oil into useful end products requires extensive processing, including reducing the boiling point of the heavy oil, increasing the hydrogen-to-carbon ratio, and removing impurities such as metals, sulfur, nitrogen and high carbon forming compounds. Examples of catalytic hydrocracking processes using conventional supported catalysts to upgrade atmospheric tower bottoms include fixed-bed hydroprocessing, ebullated- or expanded-bed hydroprocessing, and moving-bed hydroprocessing. Noncatalytic processes used to upgrade vacuum tower bottoms include thermal cracking, such as delayed coking and Flexicoking, and solvent extraction. Solvent extraction is quite expensive and incapable of reducing the boiling point of the heavy oil. Existing commercial catalytic hydrocracking processes involve rapid catalyst deactivation and high catalyst cost, making them currently unsuitable for hydroprocessing vacuum tower bottoms unless substantially diluted with lower boiling fractions, such as atmospheric tower bottoms. Most existing ebullated bed processes operate at less than 65 wt % conversion, while most fixed bed processes have less than about 25 wt % conversion.
A major cause of catalyst and equipment fouling is the undesired formation of coke and sediment, which often results when asphaltenes are heated to the high temperatures required to effect catalytic and thermal cracking. Supported catalysts used in commercial hydrocracking processes such as fixed-bed and ebullated-bed processes utilize solid supported catalysts that include clusters of catalytic sites located within pores or channels in the support material. Most heavy oil feedstocks contain a significant portion of asphaltene molecules, which are either too large to enter the pores of the catalyst support or else become trapped within the pores. Asphaltene molecules that become trapped in the pores deactivate the catalyst sites in the blocked pores. In this way, smaller asphaltene molecules can progressively block all catalyst sites, entirely deactivating the catalyst.
Moreover, larger asphaltene molecules form free radicals, just like other hydrocarbon molecules in the feedstock, but, unlike smaller molecules in the feedstock, are too large to enter the catalyst pores. Because of this, they are generally unable to react with hydrogen radicals located at the catalyst sites. As a result, the larger asphaltene free radicals are free to react with asphaltene and other free radicals in the feedstock, thereby forming larger molecules which continue increasing in size that can foul both the catalyst and the hydroprocessing equipment through the formation of coke precursors and sediment. The tendency of asphaltenes to form coke and sediment increases as the conversion level of the residuum increases due to the more strenuous conditions required to increase conversion. The undesirable reactions and fouling involving asphaltene greatly increase the catalyst and maintenance costs of ebullated-bed and fixed-bed hydrocracking processes. They also render existing commercial processes unsuitable for hydroprocessing vacuum tower bottoms and other very low quality feedstocks rich in asphaltenes.
Exacerbating the relatively low conversion levels using fixed bed hydroprocessing systems is the inability to proportionally convert the asphaltene fraction at the same conversion level as the heavy oil as a whole. Similarly, even though ebullated bed hydroprocessing systems are able to operate at substantially higher conversion levels than fixed bed systems, disproportional conversion of asphaltenes relative to the heavy oil as a whole is also problem with ebullated systems. The result of disproportional conversion is a progressive buildup of asphaltenes in the processed feedstock, with an attendant increase in the likelihood that coke and sediment will form in the reactor and other processing equipment.
Another problem, particularly acute in the case of ebullated-bed processes, involves continued free radical reaction in the catalyst free zones located (i) between the liquid recycle cup and the upper end of the expanded catalyst bed, (ii) between the plenum and distributor grid plate at the bottom of the catalyst bed, (iii) outside of solid catalysts within the expanded catalyst bed, and (iv) within the hot separator. The hydrocarbon free radicals generated at elevated temperatures within the ebullated bed are generally able to undergo hydrogenation reactions in the expanded catalyst zone as intended (except for larger asphaltene molecules, as discussed above). However, it is difficult for catalyzed hydrogenation reactions to occur within the catalyst free zones. Moreover, as product is withdrawn and sent to the hot separator, hydrocarbon free radicals continue to persist and may be further generated at high feedstock temperatures within the hot separator, which may only be about 2-4° C. (3.6-7.2° F.) less than the temperature of the feedstock in the ebullated bed. Because the hot separator includes no catalyst, free radicals tend to polymerize with each other rather than being capped by hydrogen through catalytic hydrogenation, thereby resulting in the formation of coke precursors and sediment with a high tendency for fouling of the hot separator, downstream heat exchangers, and even the vacuum distillation tower. The formation of coke precursors and sediment in the hot separator is exacerbated in the case where the feedstock includes a significant concentration of asphaltenes. Aside from equipment fouling, sediments often lead to instability of residual resid when it is used as a fuel oil.
To prevent fouling of the hot separator, the LC-Fining ebullated-bed hydrocracking reactor at Syncrude Canada in the Province of Alberta, Canada has been modified to reduce the temperature of the partially upgraded feedstock within the hot separator in order to reduce free radical formation and associated sediment formation and fouling that would otherwise occur in the absence of cooling. This is accomplished using an oil quench, in which cooler oil is pumped at elevated pressure to the entrance of the hot separator in order to reduce the temperature of the reactor product coming into the hot separator.
Another problem associated with conventional ebullated-bed hydrocracking processes is the need to carefully control the temperature and rapidly disperse the heat that accumulates within stagnant areas throughout the entire bed. Because many hydroconversion reactions are exothermic, and because heat can increase the rate of certain reactions, the formation of stagnant spots when the supported catalyst particles are not properly fluidized within the ebullated bed reactor can result in reactions that quickly get out of control. Stagnant spots of increased temperature can promote the formation of coke precursors and sediment, which can bind the catalyst particles together to form catalyst balls that are too heavy to be fluidized. Exothermic reactions tend to persist around the catalyst balls and stagnant zones. One ebullated-bed reactor actually blew up due to uncontrolled run-away reactions accelerated by stagnant zones caused by poor distribution of hydrogen, reportedly killing several workers in the vicinity of the reactor. Thermocouples are therefore typically placed throughout the ebullated bed in order to monitor and maintain an evenly controlled temperature throughout the reactor.
Fixed bed hydrocracking systems are even more prone to problems associated with overheating due to the closer proximity of the immobile solid catalyst particles. As a result, fixed bed systems must be operated using a shallow bed of catalyst only and while injecting cold hydrogen in between the catalyst beds to moderate the temperature before the partially converted feed and product are introduced to the next catalyst bed. Unlike an ebullated bed system, in which fresh catalyst is continually or periodically added and spent catalyst is continually or periodically withdrawn, the catalyst in a fixed-bed system can only be changed by entirely shutting down and cleaning the reactor. In addition, asphaltenes, coke and sediment plug up the catalyst bed, causing a drop in pressure. For the foregoing reasons, fixed-bed hydrocracking systems typically only operate for less than 8 months at lower conversion of about 15-25%, compared to ebullated-bed systems that typically have a conversion of about 50-60% for up to two years before shutdown, because of catalyst fouling that progressively deactivates the catalyst.
One attempt at obtaining the benefits of both ebullated bed and fixed bed systems is the so-called “moving bed” system in which fresh catalyst is periodically added to the top of a catalyst bed and spent catalyst is periodically removed from the bottom of the catalyst bed. This process has had little commercial success, however, due to the difficultly in actually adding and removing the catalyst while continuing to operate at the high pressures required to keep the hydroconversion process going.
In view of the foregoing, there is an ongoing need to develop improved hydroprocessing methods and systems that can be used at a commercial level to upgrade heavy oil feedstocks. There is also a need to develop hydroprocessing methods and systems that can be used to upgrade vacuum tower bottoms and other low grade heavy oil feedstocks that must now be processed at very low conversion levels to prevent equipment fouling.